The present invention relates to the field of power MOS transistors.
Conventionally, a power MOS transistor is often manufactured according to the so-called DMOS (Diffused MOS) technology and is comprised of a set of n1 identical cells which, due to recurrence, allow the DMOS transistor to stand high power. The value of n1 is for example 2500. This MOS transistor is usually designed to be serially connected to a load.
FIG. 1 shows a power MOS transistor M1 serially connected, through its source S1, to a load L. The transistor drain D1 is connected to a first terminal 1 of a supply source. The free terminal of load L is connected to the second terminal 2 of the supply source. Usually, a positive voltage VCC is applied to terminal 1, terminal 2 being grounded. Transistor M1 is rendered conductive if a control voltage VH higher than voltage VCC is applied on its gate G1 by a control source 3.
The load may get interrupted while the transistor is in the conductive state. The load can be considered as being interrupted when the current that flows therethrough becomes lower than a threshold current IA, the value of which is for example IA=3 mA. When the current flowing through transistor M1 drops to a value lower than the threshold current, the voltage drop VDB between the drain and source of this transistor decreases and becomes lower than 1 mV.
A way for determining such a load interruption could consist in measuring the voltage drop VDS. For this purpose, one usually uses a circuit comprising an auxiliary MOS transistor M2, comprised for example of n2=50 cells identical to the n1 cells constituting the power transistor, connected through its drain D2 to a resistor R, the free terminal of which is connected to terminal 1 of the supply source. The source S2 of transistor M2 is directly connected to the source S1 of transistor M1. The gate G2 of transistor M2 is directly connected to the gale G1 of transistor M1.
An operational amplifier A1 compares the voltage drop across the terminals of resistor R with a reference value VX. If IL is the current in load L, I.sub.M1 is the current in transistor M1, I.sub.M2 is the current in transistor M2, and R.sub.ON1 and R.sub.ON2 are the resistances in the linear state of transistors M1 and M2, respectively, the voltage drop V.sub.R across the terminals of resistor R will be: ##EQU1## Indeed, R.sub.ON1 and R.sub.ON2 always have low values with respect to R.
The value of R.sub.ON1 is conventionally of about a few tenths ohm, for example 0.3 ohm. Voltage V.sub.R is the voltage that is compared with a reference voltage. In practice, one does not know how to easily associate comparators and reference voltages wherein the value of the reference voltage is lower than a few tens millivolts, for example 30 millivolts.
Thus, it is deducted from relation (1) that it will not be possible to determine the current IL for a value lower than 30 mV/0.3 ohm=100 mA.
However, it is desired to detect currents of about 3 mA flowing through the load.